Optical relative motion detection devices (hereinafter referred to as the “optical navigation device”) typically utilize image correlation techniques to determine relative motion between the navigation device and a surface by capturing images of the surface as the navigation device passes over the surface or as the surface moves past the navigation device. Both the displacement and the direction of the relative motion of the navigation device with respect to the surface are determined by comparing one image with the following image.
Typically, light emitted by a source (e.g., light emitting diode or laser) within the optical relative motion detection device is reflected off a surface and is captured by a light-sensitive imager within the optical navigation device. As the optical navigation device is moved, the optical pattern detected (e.g., direct image of the surface, interferogram, speckle pattern, etc.) changes correspondingly between frames. From the changes observed between successively captured frames, the changing positions of the mouse over the surface can be determined.
For instance, existing optical navigation devices obliquely illuminate the surface to be navigated. Height variations on the surface cast shadows described by geometrical ray optics. The size and contrast of the shadow pattern images depends in part on the type of surface over which the optical navigation device is tracked. However, in some cases where a surface that would normally have sufficient surface variability or texture (on an optical scale) for use with an optical navigation device is covered with second, transparent material (e.g., glass or similar material), the optical navigation device is unable to track variations in a surface that is essentially featureless on an optical scale. That is, light reflected off the top surface of this overlying material would not show the spatial variations that can be used for optical tracking. As a result, optical navigation devices operating over a transparent layer present operational challenges.